I want to control the direction of a DC motor...I was doing it with four SPST relays which I realize is dumb. I got a 12V DPDT relay and was just wondering how to hook it up.
Using 4 SPST relays isn't necessarily "dumb"; using a DPDT relay may be "dumb" - depending on if you need to be able to turn the motor off. If you do - then a good compromise is two SPDT relays.
The main reason to use SPST relays is the fact that in many cases, you can find larger, higher-current (for the contacts) relays (and for the really large ones, they are called "contactors") easier than you can a DPDT. As long as you make sure that only the proper pair are energized to avoid shoot-thru (and smoke - or fire, depending on what you are controlling) - using four SPST relays/contactors may be what is needed.
Similarly, SPDT relays can be found with higher contact current capability (for a given price) than can an DPDT relay; and if you need an "off" position for the motor, this is a better way to do it than say, using a DPDT relay and a separate SPST relay to control power - mainly because "off" when using SPDT relays is a "brake" function (whereas the DPDT/SPST combo, off is "coast" - now, if you need coasting instead of braking, that may be the better choice).
Also - four SPST relays can give you the choice of coasting or braking for "off" - so there's that, too.